Gas turbine engines designed for propelling an aircraft at supersonic velocity typically have high specific thrust to enable relatively small engine size. Large engine size, and respective heavy weight, are undesirable for supersonic aircraft. The engine specific thrust is represented in terms of pounds of thrust per pound of airflow, and high values thereof produce high engine exhaust velocities. However, jet noise is directly proportional to exponential values of exhaust velocities, and, for a high speed civil transport aircraft operable at flight velocities of greater than about Mach 2, jet noise is proportional to the fourth order of exhaust velocity.
Government regulation limits the acceptable amount of noise which may be generated by aircraft engines during takeoff and initial climb of the aircraft. Since jet noise is directly proportional to exhaust velocity, and since it is desirable to have relatively small aircraft engines with relatively high specific thrust, exhaust velocities are substantial and effective means for reducing the noise are required which do not adversely affect the performance of the engine or decrease specific thrust.
Since high speed civil transport (HSCT) gas turbine engines are sized for propelling the HSCT aircraft at supersonic velocity, conventional variable area converging-diverging exhaust nozzles are required for efficiently operating the engine from takeoff through subsonic to supersonic aircraft velocities. The exhaust gases channeled through the exhaust nozzle also flow at supersonic velocity resulting in relatively high jet noise from the engine.
Conventional means for suppressing jet noise include mixing relatively cool ambient airflow or engine bypass airflow with the exhaust gases for reducing the velocities thereof and thusly reducing jet noise. The air is typically mixed with exhaust gases through conventional ejector means or through a coannular cooling air channel formed around a plug centerbody disposed in the exhaust nozzle. Various types of chutes are also conventionally known for mixing low velocity air with the high velocity exhaust gases.
However, conventional means for suppressing jet engine exhaust noise have varying degrees of effectiveness, and would be relatively large for a HSCT gas turbine engine application for propelling an aircraft at relatively high supersonic velocity greater than about Mach 2.